The venue for visual signal transduction, the process whereby a light stimulus is translated biochemically into an electrical signal, is the outer segment of photoreceptor cells. This system has been studied extensively and therefore a wealth of information is available. For instance, we know many of the players involved in signaling and their respective roles, and the high-resolution crystal structures of some of the signaling proteins. Despite the intense efforts and the abundance of information, a complete picture of the signaling process is still unavailable. Gaps in our understanding include critical pieces of information such as the spatial organization and the stoichiometry of proteins involved in signaling and the nature of the complexes that are formed by those proteins. Moreover, little is known about how the morphology of the outer segments is maintained and what causes their degeneration in disease and with age. In this study, we propose to develop cryo-electron tomography (cryo-ET) to study the structure of rod outer segments (ROS). Specifically we will characterize: 1) the structure of mouse ROS using cryo-ET; (2) the higher-order organization of ROS containing opsin (LRAT mice) or P23H mutant Rho associated with adRP; and (3) a high molecular weight protein that functions as a pillar holding apart membranes by a specific distance. These studies will add significantly to our understanding of the normal and disease stage organization of the rod outer segments. [unreadable] [unreadable] [unreadable]